116 



OHAS. B. WILSON. 



approach of danger can separate in two or more parts, each 

 of them capable of regeneration and so of producing an 

 entire new animal, evades this danger very effectively by 

 doing so/^ 



But even if all the fragments are not capable of regenera- 

 tion, the nemertean still has the chance that its enemy will 

 be satisfied with half a loaf, and that the posterior half. 



The actual fission of the body-walls is brought about by 

 internal phenomena which will be discussed later (see p. 118). 



The circular muscles at the severed ends contract violently, 

 and draw the margins of the rupture together so tightly that 

 the end is completely closed. It is also rounded like the true 

 anal end of the body, and this resemblance is often increased 

 by a slight emargination at the centre, due to the excessive 

 contraction of the muscles, and by the formation of a regene- 

 rating papilla in this emargination (fig. 2). 



Such a papilla is slender and almost pure white in colour. 

 At first it is difiicult to distinguish it from the true anal 

 papilla with which the body normally terminates, but it may 

 be recognised by the fact that it always possesses a very 

 broad base which fades gradually into the body- wall, while 

 the anal papilla is narrow and ends abruptly at the emargi- 

 nation. 



After closing the broken end the circular muscles go into 

 a kind of tetanus, and keep it closed until its edges are joined 

 permanently by the formation of new tissue. 



No such formation of new tissue has been observed thus 

 far in the case of the posterior fragments, but their ends 

 remain closed, and this no doubt aids them in retaining their 

 vitality for so long a time. The regeneration papilla rapidly 

 increases in length and width, but always tapers much more 

 tljan the normal body and remains light-coloured. Speci- 

 mens have been obtained in which this papilla was four or 

 five inches in length. 



Fission takes place through the intestinal caeca, since they 

 offer the point of least resistance, and it usually involves a 

 rupture of the adjacent sexual pouches. 



